Rates of reaction Flashcards
What are the requirements for an effective collision?
- Particles collide with correct orientation
- Particles have energy greater than activation energy
How does a rate of reaction graph change over time?
Initially increases: -high conc of reactants -high frequency of collisions -correct orientation -energy greater than Ea Slows down: -reactants lower conc -less frequent collisions Stops: -limiting factor is all used up -collisions stop -no more product formed
Methods for following progress of a reaction:
- Monitoring removal of reactants
- Following formation of products
How do you calculate rate of reaction from a curve graph?
- Draw tangent at steepest point of graph
- Calculate gradient of line from tangent
What are the factors affecting rate of reaction?
- Concentration
- Temperature
- Use of a catalyst
- Surface area of solid reactants
What is the definition of concentration?
Amount of substance in a defined space
What is a catalyst?
Substance that increases the rate of reaction by finding an alternate reaction pathway with lower activation energy
How does the presence of a catalyst affect the rate of reaction?
Increases the rate of reaction
What is a homogenous catalyst?
Catalyst is in the same phase or state as the reactants
How does a a homogenous catalyst work?
Catalyst reacts with reactant to form an intermediate
Intermediate breaks down to give the product and regenerates the catalyst
What are some examples of homogenous catalysts?
- Sulfuric acid for making esters
- Cl radicals in the ozone depletion
What is a heterogenous catalyst?
Catalyst has a different physical state as reactants
Usually in solids, in contact with gaseous reactions or reactant in solution
How does a heterogenous catalyst work?
Reactant molecules are absorbed onto surface of catalyst where the reaction takes place
After reaction, product molecules leave surface of catalyst by desorption
What are some examples of heterogenous catalysts?
- Fe (s) Haber Process
- Pt (s) or Rh (s) Reforming
- Ni (s) Hydrogenation of alkenes
- V2O5 (s) Making sulfur trioxide for sulfuric acid
Why are catalysts important for sustainability?
- Allow reactions to operate at lower temperature
- This reduces demand for energy
- Meaning less fossil fuels are used
- Which reduces CO2 emissions
- Slows down global warming
- This also cuts down costs
What are the main features of a Boltzmann Distribution?
- Area under curve is equal to total number of molecules
- No maximum energy for molecule (line never reaches x-axis)
- No molecules have zero energy (curve starts at origin)
What happens to the shape of a Boltzmann Distribution at higher temperatures?
- Peak is lower and shifted to the right
- Greater area shaded as more molecules have Ea
Why does rate of reaction increase at higher temperatures?
-More molecules have an energy greater than or equal to activation energy
-Higher frequency of successful collisions, so rate of reaction increases
(kinetic energy of particles also increases, but contribution is less significant)
Why does the peak lower and move to the right at higher temperatures?
- As number of molecules increases with Ea, peak lowers to COMPENSATE
- This is because AREA UNDER GRAPH MUST STAY THE SAME as this represents NO. OF MOLECULES, which cannot change
How does a catalysts change a Boltzmann Distribution?
Greater area of graph is shaded as more molecules have energy greater than or equal to activation energy as Ea is lowered
Why does a catalyst increase the rate of reaction?
- By finding an alternate reaction pathway with lower activation energy, more molecules are able to have activation energy or greater
- Therefore, there is a higher frequency of successful collisions so rate of reaction increases
How to represent effect in catalyst on Boltzmann Distribution:
- Draw one line, further right, representing Ea without catalyst
- Draw another line, to the left, and label this Ea with catalyst
- This will show a higher proportion of molecules with Ea
Why might the rate of reaction slow down even if there is a catalyt?
If the catalyst is a biological catalyst, then at too high temperatures (e.g 50*C) it will denature (active site changes conformation due to broken bonds) and therefore have no effect
What is dynamic equillibrium?
Where the rate of the forwards reaction equals the rate of the backwards reaction, in a closed system. And the concentration of reactants and products stays the same
What is Le Chatelier’s Principle?
A system will try to reverse any changes introduced in order to return to dynamic equillibrium
What are the rules for shifting the position of the equillibrium?
- If the forwards reaction is favoured, the equilibrium has shifted to the right
- If the backwards reaction is favoured, the equilibrium is shifted to the left
How does the concentration of reactants affect the position of equilibrium?
Increased concentration of reactants shifts the equilibrium to the right
How does pressure affect the position of equilibrium?
Increased pressure favours side with fewer moles of gas
How does temperature affect position of equilibrium?
Increase temperature favours the endothermic reaction
What affect does a catalyst have on the position of equilibrium?
No effect
Why does a catalyst have no effect on the position of equilibrium?
Because it increases the rate of the forwards and backwards reaction equally
brown 2N02 (g) N204 (g) colourless What will you see in terms of colour change if pressure increases?
Increased pressure will favour the forward reaction, so equilibrium shifts to the right
Colour of mixture will become paler in colour (less brown) as more N204 is being produced
Yield will increase
What is the equilibrium constant?
Can be used to determine whether more product or more reactants will be made
What is the equation for equilibrium constant?
Kc= conc. of products
- ---------------------
conc. of reactants
aA +bB cD + dD
Write an expression to work out equilibrium constant
Kc = [C]^c [D]^d
——————
[A]^a [B]^b
What are the rules for equilibrium constant?
- If Kc > 1 then product is favoured
- If Kc < 1 then reactant is favoured
What effect does temperature have on the value of Kc?
If the forward reactions is endothermic then increasing the temperature will decreases the Kc value as less product is being made
Opposite for endothermic reaction
What effect do concentration, pressure and catalyst have on the Kc value?
No effect
Value of Kc is independent of these factors
Cl2 (aq) + H2O (l) HClO (aq) + HCl
State two disturbances which would result in a decrease in the concentration of chlorine:
- Increase concentration of H2O
- Decrease concentration of products
HCOOH + CH3OH HCOOCH3 + H2O enthalpy change 0
Two disturbances that would result in no change in position of equilibrium
- Add catalyst
- Change temperature
Where can equilibrium reactions be useful?
- Cobalt chloride is used as a humidity indicator in weather instruments. It is also used to test for the presence of water (plumbers use it to test for tiny leaks in pipes)
- Haemoglobin
